Atrial synchronized, dual chamber, pacing modes, particularly, the multi-programmable, VDD, VDDR, DDD and DDDR pacing modes, have been widely adopted in implantable dual chamber pacemakers for providing atrial and ventricular or AV synchronized pacing on demand. Such dual chamber pacing modes have also been incorporated into implantable cardioverter/defibrillators (ICDs) and into right and left heart pacing systems providing synchronized right and left heart pacing for enhancing left ventricular cardiac output as described in commonly assigned U.S. Pat. No. 5,902,324.
Such pacing systems are embodied in an implantable pulse generator (IPG) adapted to be subcutaneously implanted and at least atrial and ventricular pacing or cardioversion/defibrillation leads that are coupled to the IPG. The atrial and ventricular leads each incorporate one or more lead conductor that extends through the lead body to an exposed pace/sense electrode or cardioversion/defibrillation electrode disposed in operative relation to a heart chamber. Typically, a negative-going or cathodal voltage pacing pulse is applied through a pacing path comprising a small surface area, active pace/sense electrode (also characterized as a cathode electrode) and a relatively larger surface area, return or indifferent pace/sense electrode (also characterized as an anode electrode) to pace a heart chamber.
Such leads are typically characterized as unipolar leads if they comprise only a single active pace/sense electrode and/or a cardioversion/defibrillation electrode. In the pacing context, a unipolar lead is coupled with a unipolar IPG, wherein the electrically conductive IPG housing or “can” comprises a return or indifferent pace/sense electrode or anode electrode. Unipolar pacing and sensing takes place between the lead-borne active pace/sense electrode and the housing indifferent pace/sense electrode. A bipolar lead comprises at least two lead conductors coupled to a bipolar IPG and extending to an active pace/sense electrode, typically located at the distal end of the lead body, and an indifferent pace/sense electrode, typically located on the lead body proximal to the distal active pace/sense electrode. Bipolar pacing and sensing takes place between the lead-borne active pace/sense electrode and indifferent pace/sense electrode. In the bipolar configuration, the indifferent pace/sense electrode is usually a ring-like structure, referred to as the “ring” electrode, located proximal to the distal active pace/sense electrode, by about 0.5 cm to 2.5 cm. In this context, bipolar and unipolar sensing may also be referred to as “near-field” and “far-field” sensing, respectively. (Although “far-field” usually denotes sensing outside the chamber of interest, and the unipolar signal derived from such a unipolar pace/sense electrode pair is dominated by the near-field tip electrode signal.)
A pacing IPG capable of pacing in atrial synchronized modes typically includes atrial and ventricular sense amplifiers, atrial and ventricular pace pulse generators or “amplifiers”, an operating system governing pacing and sensing functions, and components as described further herein in relation to a preferred embodiment of the invention.
In the typical dual chamber DDD pacing system, an atrial pacing (A-PACE) pulse generated by the atrial pace pulse generator is applied to the right atrial active and indifferent pace/sense electrodes to cause the right and left atria to depolarize. Similarly, a ventricular pacing (V-PACE) pulse generated by the ventricular pulse generator is applied to the right ventricular active and indifferent pace/sense electrodes to cause the right and left ventricles to depolarize. In more recently developed right and left heart pacing systems, pacing pulse generators and leads are incorporated into the pacing system to provide A-PACE and/or V-PACE pulses to the left atrium and/or ventricle.
The atrial sense amplifier is coupled to atrial active and indifferent pace/sense electrodes to detect electrical signals of the heart associated with atrial depolarizations (P-waves) and to generate an atrial sense event (A-EVENT) signal when detection criteria are met. The ventricular sense amplifier is coupled to ventricular active and indifferent pace/sense electrodes to detect electrical signals of the heart associated with ventricular depolarizations (R-waves) and to generate a ventricular sense event (V-EVENT) signal when detection criteria are met.
The pacing operating system times out various intervals from each A-EVENT, V-EVENT, A-PACE, and V-PACE to maintain synchronous depolarizations of the atria and ventricles. Such AV synchronous pacemakers that perform this function have the capability of tracking the patient's natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates. Maintenance of AV mechanical synchrony is of great importance as set forth in greater detail in commonly assigned U.S. Pat. No. 5,626,623.
Typically, the IPG operating system comprises a microcomputer controlled, digital controller/timer circuit that defines and times out a V-A interval (in DDD and DDDR modes) or a V-V interval (in VDD and VDDR modes) upon a V-EVENT or V-PACE pulse and times out an AV delay in response to an A-EVENT (in VDD, VDDR, DDD, DDDR modes) or in response to an A-PACE pulse (in DDD and DDDR modes) as well as a number of other intervals. An SAV delay is commenced by declaration of an A-EVENT, and a PAV delay is commenced upon delivery of the A-PACE pulse in certain DDD and DDDR pacing systems.
The A-PACE and V-PACE pulses are produced by the exponential discharge of respective atrial and ventricular output capacitors through the impedance loads in the atrial and ventricular pacing paths that each include a coupling capacitor, the active and indifferent pace/sense electrodes, and the patient's heart tissue between the pace/sense electrodes. In conventional dual chamber pacing systems, both the atrial and ventricular sense amplifiers are “blanked”, i.e., uncoupled, from the respective atrial and ventricular pace/sense electrode pairs during the delivery of either of an A-PACE pulse or a V-PACE pulse and for a programmed blanking period thereafter. The gains of the atrial and ventricular sense amplifiers are normally tuned for the relatively low voltages of the heart (e.g., 0.3 mV-4.0 mV for the atrial sense amplifier and 1.0 mV-20.0 mV for the ventricular sense amplifier). The significantly greater voltages of the A-PACE and V-PACE pulses (e.g., varying between 0.5 V and 8.0 V) must be blocked from the atrial and ventricular sense amplifiers.
Moreover, a residual post-pace polarization signal (or “after-potential”) remains in the pacing path due to the residual energy in the impedance load that the output capacitor is discharged into to deliver the A-PACE or V-PACE pulse. The impedance load across the output amplifier terminals comprises the impedance of the coupling capacitor, the lead conductor(s), the tissue-electrode interface impedances, and the impedance of the body tissue bulk between the active and indifferent pace/sense electrodes. The impedances of the body tissue and the lead conductor(s) may be modeled as a simple series bulk resistance, leaving the tissue-electrode interfaces and any coupling capacitors as the reactive energy absorbing/discharging elements of the total load. There are typically two tissue-electrode interfaces in a pacing path, one at the active tip electrode, and one at the indifferent ring (or IPG case or “can”) electrode. The energy stored in these interfaces and any coupling capacitors dissipates after the pacing pulse through the pacing path impedance load creating the after-potential that can be sensed at each electrode and affect the ability of the sense amplifiers to sense natural or evoked cardiac events. The tip electrode is the primary after-potential storage element in comparison to the case and ring electrodes. An indifferent ring electrode typically stores more energy than does a can electrode due to differences in electrode areas.
Most current pacemaker output circuits incorporate “fast recharge” circuitry for short-circuiting the pacing path and actively dissipating or countering after-potentials during the blanking of the sense amplifier's input terminals to shorten the time that it would otherwise take to dissipate after-potentials. The primary purposes of providing a recharge operation are to ensure that the coupling capacitor(s) is recharged to an insignificant voltage level or equilibrium prior to the delivery of the next pacing pulse through it and to allow the net DC current in the pacing path to settle to zero to facilitate sensing in the same pacing path or using one of the pace/sense electrodes of that pacing path.
Thus, it is conventional to suppress or blank both of the atrial and ventricular sense amplifiers during A-PACE and V-PACE pulses for blanking periods to avoid overloading the sense amplifier. Moreover, the sense amplifiers may abruptly sense a different potential than was present at the time of initial blanking when the blanking period expires and the sense amplifier is reconnected due to the after-potentials and electrode polarization as well as the recharge function. This can produce unwanted oversensing of artifacts resulting in false declarations of A-EVENTs or V-EVENTs. Therefore, the blanking periods in pacemaker IPGs sold by the assignee of this application are nominally set at 30 ms after delivery of an A-PACE or V-PACE, but the blanking periods may be programmed as long as 45 ms in difficult sensing scenarios. There may be additional digital blanking of the sense amplifiers to avoid sensing of evoked response or other pacing artifacts, e.g., for 150 ms to 400 ms after paced events in ICDs. Such blanking periods are characterized as an atrial blanking periods (ABP) including a post atrial pace, atrial blanking period (PAABP or PAAB) and a post ventricular pace, atrial blanking period (PVABP or PVAB) or as a ventricular blanking periods (VBP) including a post atrial pace, ventricular blanking period (PAVBP or PAVB), and a post ventricular pace, ventricular blanking period (PVVBP or PVB).
In addition, a number of sense amplifier refractory periods are timed out on atrial and ventricular sense event signals and generation of A-PACE and V-PACE pulses, whereby “refractory” A-EVENT and V-EVENTs during such refractory periods are selectively ignored or employed in a variety of ways to reset or extend time periods being timed out. Atrial and ventricular refractory periods (ARP and VRP) are commenced upon an A-EVENT or V-EVENT or generation of an A-PACE or V-PACE pulse, respectively. The ARP is typically only employed by itself during atrial demand pacing in the AAI pacing mode. In dual chamber pacing modes, the ARP commenced by the A-EVENT or A-PACE pulse extends through the SAV delay or the PAV delay until a certain time following a V-EVENT terminating the SAV or PAV delay or generation of a V-PACE pulse at the expiration of the SAV or PAV delay. This post-ventricular atrial refractory period (PVARP) is commenced by a V-PACE pulse or V-EVENT based on the understanding that A-EVENTs sensed during its time-out generally reflect a retrograde conduction of the evoked or spontaneous ventricular depolarization wave and therefore are not employed to reset an escape interval and commence an SAV delay. The duration of PVARP may be fixed or vary as a function of sensed atrial rate or pacemaker defined pacing rate, with the result that in many cases relatively long PVARPs are in effect at lower rates. A total ARP (TARP) is defined as the entire duration of the ARP and the PVARP. See, for example, U.S. Pat. No. 6,311,088. Typically the ARP and VRP are set at 300 ms, and the PVARP durations are programmable in the range of 250 ms-400 ms.
The rate-adaptive VDDR, DDIR, and DDDR pacing modes function in the above-described manner but additionally provide rate modulation of a pacing escape interval between a programmable lower rate and an upper rate limit (URL) as a function of a physiologic signal or rate control parameter (RCP) related to the need for cardiac output developed by a physiologic sensor. At times when the intrinsic atrial rate is inappropriately high or low, a variety of “mode switching” schemes for effecting switching between tracking modes and non-tracking modes (and a variety of transitional modes) based on the relationship between the atrial rate and the sensor derived pacing rate have been proposed as exemplified by commonly assigned U.S. Pat. No. 5,144,949.
In order to maximize the useful life of pacing IPGs, it is desirable that the A-PACE and V-PACE pulse energies be programmed to the minimal energies required to evoke a depolarization of the atria and ventricles (i.e., to “capture” the atria and ventricles). The minimum output pulse energy which is required to capture and thus evoke a muscular depolarization within the heart is referred to as the stimulation threshold, and generally varies in accordance with the well known strength-duration curves, wherein the amplitude of a stimulation threshold current pulse and its duration are inversely proportional. One difficulty that arises from use of the blanking and refractory periods relates to the inability to use the sense amplifiers to detect the capture or loss of capture (LOC) of the atria and ventricles.
Therefore, it has been proposed to employ additional sense electrodes and sense amplifiers or differing combinations of pace/sense electrodes or cardioversion/defibrillation electrodes to sense the evoked response to a V-PACE or A-PACE as described in commonly assigned U.S. Pat. Nos. 5,331,966 and 5,683,431. A subcutaneous electrode array (SEA) formed on the surface of the IPG housing is proposed in the '966 patent for sensing the “far field” EGM at a distance from the heart along vectors selected from the electrodes of the SEA. The far field EGM is employed for a variety of reasons as set forth in the above-referenced '966 patent. The '966 patent also describes a number of other sensing schemes in the prior art for sensing the electrical activity of the heart for determining LOC or other reasons including the following.
U.S. Pat. No. 3,949,758 relates to a threshold-seeking pacemaker with automatically adjusted energy levels for pacing pulses in response to detected LOC, and describes separate sensing and pacing electrodes, which are each utilized in unipolar fashion with a third common electrode having a comparatively larger dimension, to reduce residual polarization problems.
U.S. Pat. No. 3,977,411 discloses a pacemaker having separate sensing and pacing electrodes that are each utilized in unipolar fashion. The sensing electrode comprises a ring electrode having a relatively large surface area (i.e., between 75 to 200 mm2) for improved sensing of cardiac activity (R-waves), and is spaced along the pacing lead approximately 5 to 50 mm from the distally-located tip electrode used for pacing.
U.S. Pat. No. 3,920,024 discloses a pacemaker having a threshold tracking capability that dynamically measures the stimulation threshold by monitoring the presence or absence of an evoked response (R-wave). Various electrode configurations are illustrated in FIGS. 1B and 9A-9F for purposes of sensing the evoked response, including sensing is between an intracardiac electrode and a reference electrode that is spaced some distance away from the heart or sensing between intracardiac electrodes.
U.S. Pat. No. 4,305,396 also relates to a rate-adaptive pacemaker wherein the output energy is automatically varied in response to the detection or non-detection of an evoked response (R-wave) and the detected stimulation threshold. It is stated to be preferred to use the same electrode for both pacing and sensing, such as a unipolar or bipolar system wherein there is at least one electrode located in the ventricle, but suggests that other lead designs may be utilized such that the sensing and pacing electrode are separate.
U.S. Pat. No. 4,387,717 relates to a pacemaker having a separate (i.e., non-pacing) electrode element, implanted near or in direct contact with the cardiac tissue, and positioned relative to the pacing electrodes (i.e., unipolar pacing from “tip” to “can”) to provide improved P-wave and R-wave sensing with minimal interference from the pacing electrodes. The “can” functions as an indifferent electrode for sensing in combination with the separate electrode element. The separate sensing electrode is spaced from the pacing electrodes to minimize cross coupling and interference from the pacing stimulus and after-potentials. The separate sensing electrode comprises an extravascular metallic plate having a comparatively large surface area in one embodiment. In another embodiment the separate sensing electrode comprises a cylindrical metal ring mounted on the insulated pacing lead between the pacemaker and the “tip” electrode, and is described as being located along the lead to permit positioning the sensing electrode either within the heart, externally on the heart wall, or in some remote location in the vascular system away from the heart.
U.S. Pat. No. 4,585,004 relates to an implantable cardiac pacing and monitoring system, wherein the pace/sense electrodes are electrically separate from an auxiliary sense electrode system. The auxiliary sense electrode system comprises a transvenous data lead with ring electrodes for sensing located in the right ventricle (approximately 1 cm from the pacing tip electrode for R-wave sensing) and in the right atrium (approximately 13 cm from the tip electrode to be in close proximity with the S-A node), both ring electrodes being used in conjunction with the pacemaker can in unipolar sensing fashion.
U.S. Pat. No. 4,686,988 relates to a dual chamber pacemaker having atrial and ventricular endocardial leads with a separate proximal ring electrode coupled to a P-wave or R-wave sensing EGM amplifier for detecting the atrial or ventricular evoked response to atrial or ventricular stimulation pulses generated and applied to other electrodes on the endocardial lead system. The auxiliary lead system thus resembles the '004 patent.
U.S. Pat. No. 4,549,548 discloses a programmable DDD pacing system in which the selection of pace/sense electrodes is changed during each pacing cycle to optimize the choice of unipolar and bipolar atrial and ventricular operations. U.S. Pat. Nos. 4,759,366 and 4,858,610 relate to evoked response detector circuits that also employ fast recharge in at least one separate sensing electrode in either unipolar or bipolar electrode configurations in either or both the atrium and ventricle. The cardiac pacing systems function as unipolar and bipolar systems at different steps in the operating cycle. In the '610 patent, a separate electrode on the connector block of the IPG can is suggested for use as the reference electrode anode rather than the metal case itself if the case is employed as the reference electrode for the delivery of the stimulation pulse. In the '366 patent, the detected evoked response is used in an algorithm for adjusting the pacing rate.
U.S. Pat. Nos. 4,310,000, 4,729,376, and 4,674,508 also disclose the use of a separate passive sensing reference electrode mounted on the IPG connector block or otherwise insulated from the pacemaker case in order to provide a sensing reference electrode which is not part of the stimulation reference electrode and thus does not have residual after-potentials at its surface following delivery of a stimulation pulse. The aforementioned '000 patent suggests various modifications to the passive sensing reference electrode depicted in its drawings, including the incorporation of more than one passive sensing reference electrode provided on or adjacent to the IPG can, positioned as deemed necessary for best sensing, and connected to one or more sense amplifiers. No specific use of the additional passive sensing reference electrodes is suggested, although the single passive sensing reference electrode is suggested for use with a sense amplifier to detect both capture and spontaneous atrial or ventricular electrical events in a dual chamber pacing system.
Moreover, it has been proposed in the prior art to automatically select among pacing and sensing electrode pairs during the cardiac cycle or in response to a determination that lead impedance is unacceptable (which may arise from a lead fracture or electrode dislodgement or the like). See, for example, U.S. Pat. Nos. 4,958,632, 5,003,975, and 5,755,742 and the above-referenced '548 patent. According to the '548 patent, the selection of unipolar or bipolar mode of operation is based on a determination for monitoring the amplitude of sensed heartbeat signals to determine whether the sensing operation would be performed better in the unipolar or the bipolar mode. This is directed to a determination of heart performance vis-à-vis the leads involved so as to control the selection of unipolar or bipolar sensing.
Thus, considerable effort has been expended in providing systems and methods for overcoming the limitations on sensing imposed by delivery of a pacing pulse across a pair of pace/sense electrodes for a variety of purposes, including detection of LOC and determination of pacing thresholds, determination of lead impedance, and selection of the optimal pacing and sensing electrode pairs. Despite these improvements, pacing systems still employ the above-described atrial and ventricular blanking functions.
Disruption of AV electrical and mechanical synchrony frequently arises due to the spontaneous depolarization of the ventricles triggered at an ectopic site in one of the ventricles. Such a spontaneous ventricular depolarization that is not associated with a prior atrial depolarization is characterized as a premature ventricular contraction (PVC). Many of the problems resulting from the occurrence of a PVC in a patient with a dual chamber pacemaker are described more fully in U.S. Pat. Nos. 4,788,980 and 5,097,832.
PVCs that occur during the V-A interval following a prior detected R-wave or delivery of a V-PACE pulse are usually sensed as V-EVENTs that restart the V-A interval. PVCs that occur during the time-out of the AV delay and following time-out of the PAVBP are indistinguishable from sinus ventricular depolarizations that are conducted from the AV node through the Bundle of His. The resulting V-EVENT inhibits delivery of the V-PACE, and the V-A interval is commenced.
As noted above, after-potentials on the ventricular pace/sense electrodes at time-out of the PAVBP can erroneously be detected and result in declaration of a V-EVENT by the ventricular sense amplifier. The pacing system will not provide appropriate ventricular pacing to a patient's heart having AV block if electrical noise or other signals are mistakenly sensed by the ventricular sense amplifier as V-EVENTS during time-out of the AV delay. The questionable nature and consequences of mistakenly detecting V-EVENTs has led to the adoption of the practice of delivering a ventricular safety pace (VSP) pulse at a fixed time, typically 110 ms, following delivery of an A-PACE. In other words, a VSP pulse is delivered to the ventricular pace/sense electrodes if a V-EVENT is declared between the time-out of the PAVBP and a 110 ms VSP window following delivery of an A-PACE pulse. This 110 ms VSP window is often denoted the cross talk window. The 110 ms VSP window length is shorter than the normal AV conduction time in humans, so any V-EVENT declared within the VSP window is unlikely to be due to true AV conduction. The delivered VSP pulse captures the ventricles if the V-EVENT was due to cross talk, that is, sensing of the residual A-PACE energy afterpotentials. The delivered VSP pulse will not capture the ventricles if the V-EVENT reflects a PVC, because the ventricles will be refractory at that time. Thus, faced with this uncertainty, a VSP pulse is delivered at time-out of the VSP window or delay so as to ensure that the ventricles are truly contracting at a safe time after delivery of the A-PACE pulse. The VSP function is a programmable feature of prior art pacing systems that may be programmed off by the physician if desired. One form of VSP operation is set forth in U.S. Pat. No. 4,825,870, for example.
However, it frequently happens that the depolarization wavefront of a PVC reaches the pace/sense electrodes during the PAVBP, and the ventricular sense amplifier does not detect the R-wave. The after-potentials from the PVC wavefront may not be strong enough at the ventricular pace/sense electrodes to trigger a V-EVENT at time-out of the ventricular blanking period. Thus, a V-PACE pulse may be delivered at the time-out of the AV delay. The AV delay may be programmed to be long enough so that the V-PACE is delivered during the vulnerable period of the ventricles. The vulnerable period occurs during the T-wave repolarization of the ventricle (approx. 250 ms-400 ms). During the vulnerable period, there is a dispersion of refractoriness where some cardiac cells are repolarized while others are still refractory. Additional stimulation during this time has a higher likelihood of initiating a tachyarrhythmia than during periods where the cardiac cells are either completely refractory or completely repolarized.